1. Field of the Invention
This invention relates to a travelling-wave tube, and more particularly a coupled cavity type slow-wave structure for use in a high power travelling wave tube.
2. Description of the Prior Art
In slow-wave structures of the type used in a travelling-wave tube, the propagating speed of an input signal wave is slowed down to make it comparable to that of electron beams passing through the slow-wave structure, thereby bringing about the interaction between an input signal wave and an electron beam. In this connection, there arises a problem that dimensional uniformity or accuracy of the slow-wave structure has a significant effect upon its output characteristics.
This is particularly true with a coupled cavity type slow-wave structure for use in a high power travelling-wave tube, and thus difficulties are encountered in the manufacture of the slow-wave structure which may satisfy the aforesaid requirements. As has been described, the partition walls are respectively provided with central apertures adapted to pass electron beams therethrough and with coupling slots which are located in the neighborhood of the central apertures and are adapted to effect electromagnetic coupling of the adjoining cavities, while the partition walls are periodically arranged at a given spacing within a circular waveguide. Upon brazing, however, thermal deformation tends to take place in the partition walls due to the heat cycle of the brazing operation. Particularly, non-symmetric configurations of the partition walls due to the presence of coupling slots are unfavorable for avoiding deformation of the partition walls. On the other hand, accurate axial alignment of the central apertures in the partition walls is required to assure passage of the electron beams therethrough. However, the use of a jig or tool for assuring and/or correcting the axial alignment of central apertures leads to deformation of partition walls, because the partition walls are formed of a material which softens due to the heat arising from brazing.
In addition, there is another problem in that the temperature at the partition walls is increased due to the ohmic loss of high frequency current passing through the partition walls constituting the major portion of the slow wave structure, even when the tube is brought into operation. This temperature rise in the partition walls causes deformations in the partition walls, presenting inaccurate dimensions therefor. The failure to achieve and maintain accurate dimensions in turn leads to the failure in the impedance matching of the slow-wave structure with an external waveguide to supply through an input signal wave or extract an output signal wave, as well as impedance matching within the slow-wave structure itself, resulting in inaccurate responsiveness or relationship of an output signal to an input signal, and thereby presenting unfavorable results.
Still additionally, the prior art slow-wave structure poses a shortcoming of allowing no possibility of repair in the event of defective characteristics, because the slow wave structure in its entirety constitutes a vacuum envelope.
A yet another problem encountered with the prior art travelling-wave tube is that the outer dimensions of a coupled cavity type slow-wave structure are larger than the outer diameter of a helix type slow-wave structure, requiring the use of an electromagnet for obtaining a magnetic flux density of a magnitude required for focussing electron beams. This unavoidably leads to an extremely large increase in the outer diameter of the travelling-wave tube, as disclosed in a paper entitled "The Ground Station High-Power Travelling-Wave Tube" by R. J. Collier, et al., published in THE BELL SYSTEM TECHNICAL JOURNAL, July issue, 1963, pages 1829 to 1861.